US President Donald Trump on Tuesday rejected a massive COVID-19 economic relief package passed by Congress, branding it "a disgrace" in an act of political brinkmanship less than a month before he must leave office and when millions of Americans are suffering fallout from the pandemic.

Trump dropped the bombshell via a pre-recorded statement made in the White House and sent out on Twitter.

It came just a day after his Republicans and the Democrats finally agreed overwhelmingly to a $900 billion bill meant to throw a lifeline to businesses and people struggling to keep heads above water.

In his address, Trump said he would refuse to accept the bill as it is and demanded changes, notably a big increase in the proposed $600 direct payments to less well-off ­Americans.

"I am asking Congress to amend this bill and increase the ridiculously low $600 to $2,000, or $4,000 for a couple," he said, referring to relief checks.

Tapping into his nationalist "America First" brand, Trump also castigated measures added onto the bill during complex negotiations that would provide funding for projects benefiting US partners abroad and other non-COVID-19 related items like the environment.

"It really is a disgrace," he said. "I'm also asking Congress to immediately get rid of the wasteful and unnecessary items from this legislation, and just send me a suitable bill."

Trump has not yet received the bill and he did not explicitly say he would not sign. 

If he actually vetoed the package, Congress would almost certainly quickly override that, given the bipartisan support.

In what is widely expected to be only a portion of a slew of presidential pardons and commutations issued in the dying days of the administration, the White House also announced late Tuesday that 20 people had been selected.

They included two convicted in special counsel Robert Mueller's Russia probe into Trump campaign ties to Moscow and four men convicted in connection with the mass killing of 17 Iraqi civilians. 

The COVID-19 package is wrapped into a $2.3 trillion, almost 5,600-page "coronabus" bill that includes a so-called omnibus bill to fund the government for the coming year.

A congressional override of a veto would mark an embarrassing defeat for Trump, who is spending his final weeks in office before the January 20 inauguration of Democrat Joe Biden by pursuing unprecedented attempts to try and get the election results overturned.

However, until he has the bill on his desk he has no need to veto. 

Trump's motives in picking the fight with Congress are intertwined with his extraordinary ongoing struggle to overturn the November 3 election.